Current differential relay apparatuses for protecting power lines are apparatuses that recognize and detect with high sensitivity an internal fault or an external fault in a section subjected to a protection using a current of each terminal of the power line. Example configuration of a current differential relay apparatus and ratio differential characteristic thereof will be explained with reference to FIG. 16. That is, a current differential relay apparatus 100, includes an input unit 110, a receiver unit 120, a computing unit 130, an operation-level calculating unit 140, a suppression-level calculating unit 150, and ratio-differential-characteristic determining unit 160.
The current differential relay apparatus 100 as explained above recognizes an internal fault or an external fault as follows. First of all, the computing unit 130 calculates an electrical quantity based on a local-terminal current input from the input unit 110 and an opposing-terminal current from a counterpart terminal received by the receiver unit 120. The operation-level calculating unit 140 obtains a vector sum of the currents of respective terminals as an operation level Id based on the calculated electrical quantity, and the suppression-level calculating unit 150 calculates a scalar sum of the currents of the respective terminals as a suppression level Ir. The ratio-differential-characteristic determining unit 160 determines activation/deactivation of the ratio differential characteristic based on the operation level Id and the suppression level Ir.
FIG. 16 illustrates a two-terminal system and the current differential relay apparatus at a terminal A with functional blocks in order to simplify the explanation. The operation level Id remains same in the case of the systems of equal to or greater than three terminals, but the suppression level Ir is obtained based on the scalar sum of the currents of respective terminals.
As illustrated in FIG. 16, the ratio differential characteristic is defined by a set of two characteristics: a small-current range characteristic Df1; and a large-current range characteristic Df2. The small-current range characteristic Df1 is a characteristic for enabling detection even if the fault current is small when an internal fault occurs. The large-current range characteristic Df2 is a characteristic for avoiding an operation at an increased error current of a current transformer (hereinafter, referred to as a CT) when a large current flows through at the time of an external fault. In the illustrated case, when both small-current range characteristic Df1 and large-current range characteristic Df2 are activated, it becomes an output by the current differential relay apparatus 100. (Japanese Patent Application Publication No. 2002-17037 and JP 2000-224755 A)
Regarding the above-explained large-current range characteristic Df2, it is necessary to shift such a characteristic to the right to some level in consideration of the maximum load current, thereby expanding the operation range. Accordingly, when a CT which is saturated even if a current is small is used, a saturation occurs in the small-current range, which may be an unnecessary operation. Conversely, if the large-current range is excessively narrowed down, when the CT is saturated due to an internal fault, or when an internal fault bringing about a resistance occurs, an operation may become unstable.
An embodiment provides a current differential relay apparatus which includes: an operation-level calculating unit that calculates an operation level Id which is a vector sum of a local-terminal current and a received opposing-terminal current; a suppression-level calculating unit that calculates a suppression level Ir which is a scalar sum of the local-terminal current and the opposing-terminal current; and a ratio-differential-characteristic determining unit that determines an operation in a ratio differential characteristic based on the operation level Id and the suppression level Ir, and which further has following technical features.
(1) With relationships a1≦a2 and b1≧b2 being satisfied where a1, a2, b1 and b2 are real numbers, a first range determining unit which determines that a first range is activated when Id >a1Ir+b1 is satisfied; a second range determining unit which determines that a second range is activated when Id>a2Ir+b2 is satisfied.
(2) An operation determining unit which determines that an operation is made in the ratio differential characteristic when both first range and second range are activated and which continues the operation until an operation in the first range is reset.
Another embodiment provides a current differential relay apparatus which includes: an operation-level calculating unit that calculates an operation level which is a vector sum of a local-terminal current and a received opposing-terminal current; a suppression-level calculating unit that calculates a suppression level which is a scalar sum of the local-terminal current and the opposing-terminal current; and a first ratio-differential-characteristic determining unit that determines an operation in a ratio differential characteristic based on a first relational expression, and which further includes following technical features.
(1) A maximum-terminal-current selecting unit that selects a current value of a maximum-current terminal among terminals including the local terminal.
(2) A change-level determining unit which determines that a change level of the current value of the maximum-current terminal selected by the maximum-terminal-current selecting unit is equal to or greater than a predetermined value.
(3) A second ratio-differential-characteristic determining unit that determines an operation in a ratio differential characteristic based on a second relational expression.
(4) An external fault determining unit that determines an occurrence of an external fault based on determination results by the change-level determining unit and the second ratio-differential-characteristic determining unit.
(5) The first ratio-differential-characteristic determining unit includes a plurality of ratio differential characteristics with different difficulties for an operation changed in accordance with a determination result by the external fault determining unit.
Another embodiment provides a current differential relay apparatus which includes: an operation-level calculating unit that calculates an operation level which is a vector sum of a local-terminal current and a received opposing-terminal current; a suppression-level calculating unit that calculates a suppression level which is a scalar sum of the local-terminal current and the opposing-terminal current; and a first ratio-differential-characteristic determining unit that determines an operation in a ratio differential characteristic based on a first relational expression, and which further includes following technical features.
(1) A maximum-terminal-current selecting unit that selects a current value of a maximum-current terminal among terminals including the local terminal.
(2) A change-level determining unit which determines that a change level of the current value of the maximum-current terminal selected by the maximum-terminal-current selecting unit is equal to or greater than a predetermined value.
(3) A second ratio-differential-characteristic determining unit that determines an operation in a ratio differential characteristic based on a second relational expression.
(4) An external fault determining unit that determines an occurrence of an external fault based on determination results by the change-level determining unit and the second ratio-differential-characteristic determining unit.
(5) An operation-level determining unit that determines a magnitude relation between the current value of the maximum-current terminal and the operation level.
(6) The first ratio-differential-characteristic determining unit includes a plurality of ratio differential characteristics with different difficulties for an operation changed in accordance with a determination result by the external fault determining unit or a determination result by the operation-level determining unit.